The present invention relates to a method of maintaining a given content of a substance in an aqueous solution where it is continuously consumed and where the redox potential is a function of the concentration of this substance. The substance is added in such manner that a set value of the redox potential is maintained.
It is common knowledge to utilise for analyses and in examining the course of chemical reactions measuring of the redox potential in aqueous solutions (see, for instance, Ullman, "Encyklopadie der technischen Chemie", 4th Edition, Vol. 3, pp 261-262; Vol. 5, pp. 926-929). In most cases, the potential between an inert electrode, e.g. platinum, and a reference electrode, e.g. calomel (Hg, Hg.sub.2 Cl.sub.2, KCl) or silver/silver chloride, is measured. The redox potential in an aqueous solution is a measure of the ability of the components to oxidise or reduce relative to the reference electrode. Thus, the measured potential is dependent on the concentrations of the oxidising or reducing substances in the solution, and the interrelations generally are logarithmic. It has been found that, mostly, the interrelation of the redox potential E and the content x of an oxidising or reducing agent approximately satisfies the following equation: EQU E=G+F(x) (I)
F(x) is a function that in most cases can be written as T.multidot.k.multidot.ln x, wherein k is a constant and T is the absolute temperature. Thus, the derivative dF/dx will be T.multidot.k/x, the value of which asymptotically approaches 0 when x assumes a high value. G is independent of x but is influenced by other parameters, such as temperature and the contents of other components. Mostly, G is influenced considerably by the pH of the solution.
FIG. 1 illustrates the redox potential as a function of the content of hydrogen peroxide in 20% nitric acid, FIG. 2 illustrates the redox potential as a function of the content of hypochlorite ions in water at pH 10, and FIG. 3 illustrates the redox potential as a function of the content of sodium sulphite at pH 9.5. In all cases, use was made of a platinum electrode and a silver/silver chloride electrode for the measuring. As shown in the Figures, the curves level out at high contents. There are also a large number of other systems with similar redox curves, e.g. chromic acid, permanganate in acid solution, and hydrazine at high pH values.
Since the redox curves are influenced by many factors other than the concentrations that it is of essence to control, redox measuring is, in practice, not much used for process control. Although it is theoretically possible to compensate for changes in temperature and pH, it is in practice often difficult to execute correct pH measurings, especially in particle-containing solutions, or in extremely acid or alkaline solutions. Furthermore, the measuring electrodes used change in course of time, e.g. by the formation of coatings, which also affects the measured value.
GB, A, 1,280,412 discloses redox control in the regeneration of copper chloride solutions for etching. However, the shifts of the redox curves are not touched upon in this patent.
EP, A2, 267,166 discloses a method for controlling the addition of hydrogen peroxide to a solution of nitric acid in such manner that a set value of the redox potential is maintained. In the solution, the hydrogen peroxide is consumed by HNO.sub.2 being oxidised to HNO.sub.3, thus preventing the emission of nitrous gases. The addition of hydrogen peroxide is adjusted in such manner that the emission of NO.sub.x, i.e. NO and NO.sub.2, will be as low as possible, without excessive consumption of hydrogen peroxide. The redox system is characterised in that the redox potential curve shows a very distinct maximum when the hydrogen peroxide content is zero, which is used in the control. Since the curve is very steep at the maximum, uncertainty as to the redox potential only causes a negligible error in the hydrogen peroxide content. Thus, curve shifts depending on pH, temperature or the like are no major problem. The technique described in this patent functions satisfactorily in many cases, e.g. in the treatment of pickling baths in the metal industry. However, this patent provides no method for adjusting and maintaining for some time a constant hydrogen peroxide content in an aqueous solution, outside the steep section of the redox curve, where an insignificant error in the redox potential causes a considerable error in the concentration.